This invention relates to a stepping motor driving circuit used for, for example, a serial printer device.
For a conventional stepping motor driving circuit, use has been made of a closed-loop type constant current chopper driving circuit which, in order to improve the torque characteristic when the stepping motor is driven at high speeds, detects electric current through the coil of the stepping motor and controls electric current through the coil on the basis of the result of detection.
FIG. 1 shows a conventional closed-loop type constant current chopper driving circuit, FIG. 2 is a block circuit showing a constant current chopper driving circuit in detail, and FIGS. 3A through 3E are timing charts of the constant current chopper driving circuit.
In FIG. 1, four-phase stepping motor 11 includes four coils M1 through M4. The excitation current through coils M1 and M2 is controlled by transistors T1 and T2. Input terminals 12 and 13 are connected to the bases of transistors T1 and T2, respectively. When high-level signals are input to input terminals 12 and 13, transistors T1 and T2 are turned ON, causing coils M1 and M2 to be magnetically excited.
The level of electric current through coils M1 and M2 is detected as a voltage drop across current detection resistor R1 and the detected current level is input to constant current control circuit 14. Constant current control circuit 14 includes comparator 14.sub.1 , as shown in FIG. 2. The input voltage level and reference voltage level are compared with each other at comparator 14.sub.1 in constant current control circuit 14. For the input voltage level lower than the reference voltage level, the output of comparator 14.sub.1 is made at a low level and thus transistor 14.sub.2 is not turned on. As a result, a high-level (+V.sub.CC) signal is supplied to the base of transistor T5 and transistor T5 is rendered ON, supplying electric current to coils M1 and M2. For the input voltage level higher than the reference voltage level, on the other hand, comparator 14.sub.1 delivers a highlevel signal to transistor 14.sub.2 to cause the latter to be rendered conductive. As a result, a low-level signal is supplied to transistor T5, causing the latter to be rendered OFF and thus stopping the supply of electric current to coils M1 and M2.
Similarly, transistors T3 and T4 are connected to coils M3 and M4, respectively, and to current detection resistor R2, and a junction between transistors T3 and T4 is connected through current control circuit 17 to transistor T6. Input terminals 15 and 16 are connected to transistors T3 and T4, respectively.
Flywheel diodes HD1 and HD2, grounded at the negative side of a power supply, are connected one to common terminal 18 between coils M1 and M2 and one to common terminal 19 between coils M3 and M4. With transistors T5 and T6 OFF, current paths are established as HD1 .fwdarw.M1 .fwdarw.T1 .fwdarw.R1 .fwdarw.HD1, HD1 .fwdarw.M2 .fwdarw.T2 .fwdarw.R1 .fwdarw.HD1, HD2 .fwdarw.M3 .fwdarw.T3 .fwdarw.R2 .fwdarw.HD2, and HD2 .fwdarw.M4 .fwdarw.T4 .fwdarw.R2 .fwdarw.HD2.
Coils M1 through M4 are connected through zener diode ZD to flyback diodes BD1 through BD4 which are connected at the positive terminal of the power supply.
Diodes BD1 through BD4 form current paths due to the counter electromotive forces which are caused when transistors T1 through T4 change from their ON state to their OFF state. For coil M1, for example, the current path is formed as T5 .fwdarw.M1 .fwdarw.BDl .fwdarw.ZD .fwdarw.T5, at which time zener diode ZD causes the disappearance of the current resulting from the aforementioned counter electromotive force.
The operation of the constant current chopper circuit of FIG. 1 will be explained below with reference to FIGS. 3A through 3E.
FIGS. 3A through 3D show input signals S1 through S4 supplied to the bases of transistors T1 through T4. FIG. 3E shows the waveform of current Il through coil M1. When the signal S1 (FIG. 3A) becomes high, transistor T1 is turned ON, thus exciting coil M1 in four-phase stepping motor 11. At this time, the level of current Il flowing through coil M1 gradually rises, as shown in FIG. 3E, and the level of voltage across detection resistor R1 is raised. The raised voltage is supplied to comparator 14.sub.1 in constant current control circuit 14 where it is compared with the reference voltage level. When the voltage level is lower than the reference voltage level, i.e., the level of the current through coil M1 is lower than the set level, comparator 14.sub.1 delivers a low-level output signal to the base of transistor 14.sub.2 and thus transistor 14.sub.2 is not turned ON. Accordingly, transistor T5 is turned ON, since a high-level signal is supplied to transistor T5.
When the level of current through coil M1 is higher than the reference level, comparator 14.sub.1 supplies a highlevel signal to the base of transistor 14.sub.2 and hence transistor 14.sub.2 is rendered on. On the other hand, transistor T5 is turned OFF, since a low-level signal is supplied to transistor T5.
The aforementioned operation is repeated during the high-level periods of the signal, S1 so that the excitation current through coil M is maintained constant. With the signal S1 at a low level, transistor T1 is cut off, so that the level of current through coil M1 is gradually reduced to zero.
The same operation is performed for coils M2 through M4 and, in this way, coils M2 through M4 are sequentially excited, thus driving stepping motor 11.
According to the conventional stepping motor driving circuit as set out above, the closed-loop chopper driving circuit detects the level of current through the coil in the stepping motor and controls the current in the coil on the basis of the result of detection, whereby it is possible to maintain the excitation current in the coil at a constant level and thereby stabilizing the torque characteristic when the stepping motor is driven at fast speed.
In the closed-loop chopper driving circuit, however, the resistor of the current detection circuit increases as the size of the stepping motor driving circuit becomes greater, resulting in a greater error in the level of the resistor.
As shown in FIG. 2, a signal with a saw-tooth waveform is supplied to the input of comparator 14.sub.1. However, a noise problem arises due to an oscillation caused when the signal frequency exceeded a given level. Furthermore, the current control circuit becomes complex, requiring a greater number of components. As a result, such a circuit occupies a greater area while, at the same time, adding to the manufacturing costs.